LONDON, England (CNN) --As the body of UK serial killer Harold Shipman, found hanged in his prison cell, was taken away for an autopsy, relatives told of their anger.

They said they felt cheated that the man known as "Dr. Death" took to his grave the motive for killing at least 215 people -- mostly unsuspecting middle-aged and elderly women patients killed with deadly diamorphine injections.

Shipman was found in his cell at Wakefield prison, northern England, by prison staff early Tuesday, the day before his 58th birthday. (Full story)

He had hanged himself using bed sheets from the window bars in his cell, a Prison Service spokeswoman told the UK Press Association.

Relatives said they were upset Shipman showed no remorse and maintained his innocence even after his January 2000 conviction at Preston Crown Court for 15 murders which a judge described as "shocking beyond belief."

Later a public inquiry heard he had murdered more than 215 patients, mostly elderly women, in a 23-year killing spree and was suspected of killing another 45.

"He's a complete coward," Royal Air Force lieutenant Danny Mellor, 54, whose 73-year-old mother Winifred was one of Shipman's last three victims, told Reuters. "He has shown his cowardice in the most graphic manner." (Relatives angry)

Ann Alexander, a solicitor who had represented many of the victims' families told CNN her clients felt "very cheated."

She said many of them thought that he would eventually talk about what he did "and, most importantly, why he did it."

Shipman was planning an appeal against his conviction for 15 murders, his lawyer revealed Tuesday.

Giovanni di Stefano expressed his surprise that Shipman, "who at last had a chance" to appeal, "suddenly commits suicide -- a day before his birthday" and called for a "proper and full inquiry."

He added that the UK Prison Service should have learnt its lesson since "House of Horrors" UK serial murder suspect Fred West committed suicide while awaiting trial.

He told Sky TV: "For the alleged victims, this will not be seen as justice.

"Dr Shipman had never ever accepted his guilt and he's never ever admitted culpability in any of the murders that he's convicted.

"Something is not really quite right there... A person who has a ground of appeal, a precedent in the Court of Appeal to allow themselves to appeal, suddenly commits suicide a day before his birthday?

"A person who is supposed to be the biggest mass murder in the United Kingdom, with so many deaths on his hands allegedly ... is left unattended in this way?"

Shipman's former surgery in Hyde, Greater Manchester

Later Shipman's body was transported from Wakefield Prison to the Medico-Legal Centre in Sheffield for a post-mortem.

He had been on suicide watch briefly earlier in his sentence at Manchester prison and on being moved to Frankland prison in February 2000, but never since he arrived at Wakefield in June 2003.

But he had been a difficult prisoner in Wakefield jail and in December he had his privileges removed, including having a television in his room and dispensing with prison uniform because of his "attitude and behaviour" towards staff and prison officers.

Harry Fletcher, assistant general secretary of the National Association of Probation Officers, Shipman's death in a prison cell was "extremely worrying."

Fletcher told PA he had learned from sources that Shipman was not deemed a suicide risk. He was still checked frequently but irregularly -- roughly every hour.

He said: "It's extremely worrying that one of the most high profile prisoners in the country has committed suicide."

Mark Leech, founder of ex-offenders' charity Unlock and editor of the Prisons Handbook, said of Shipman's death: "I'm not in the least surprised.

"Shipman was a very skilled general practitioner and he would have known the signs and symptoms that prison staff are trained to look for in terms of suicidal prisoners and it seems he successfully hid those symptoms.

"I don't think there is any criticism that can be laid against the prison system in this case."

An investigation into the death will be carried out by the UK Prisons and Probations Ombudsman Stephen Shaw, who takes over full responsibility for such inquiries soon.